Known from the state of the art are field devices, which are used in industrial plants. Field devices are often applied in process automation technology as well as in manufacturing automation technology. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. Thus, field devices are used for registering and/or influencing process variables. Serving for registering process variables are measuring devices, i.e. sensors. These are used, for example, for pressure- and temperature measurement, measuring conductivity, flow measurement, etc. and register the corresponding process variables, pressure, temperature, conductivity, pH-value, fill level, flow, etc. Used for influencing process variables are actuators. These are, for example, pumps or valves, which can influence the flow of a liquid in a pipe or the fill level in a container. Besides the above mentioned measuring devices and actuators, the terminology, field devices, also includes remote I/Os, radio adapters, and, generally, devices, which are arranged at the field level.
A large number of such field devices are produced and sold by the Endress+Hauser group of companies.
Many field devices for process automation are exposed to various environmental influences, dependent on their location of use. In different branches of industry, thus, different influences reign, which require different protective measures.
A common solution is to use housings of metal. These are manufactured, for example, of VA steel, aluminum, and the like. For purposes of corrosion prevention, these metal housings are provided with various coatings. This is, however, in given cases, a more expensive method step.
Likewise housings of plastic are offered. These have from the corrosion point of view considerable advantages compared with metal housings. They have, however, the disadvantage that they are inferior compared with the mechanical robustness of metal housings. For manufacturers of field devices, the result is that devices must be offered both with metal housings as well as also with plastic housings.
For the electronic circuits accommodated in the housings, however, there result therefrom, as regards EMC-protection, considerable differences, which come down to differently required protective measures for metal and plastic material and result in considerable extra developmental effort. In the state of the art, plastic housings are provided with a thin metal layer on the housing interior, applied by means of various methods, such as, for example, vapor deposition, lacquering, sputtering, galvanizing, etc., in order to form a Faraday cage. These layers are, however, very thin and do not provide any additional mechanical stability for the housing. Moreover, these manufacturing processes can be costly.